The "magnetoresistance" (MR) of a material is the resistance R(H) of the material in an applied magnetic field H minus the resistance R.sub.o in the absence of an applied field. This resistance difference .DELTA.R (=R(H)-R.sub.o) is typically normalized (e.g., by dividing by R(H)) and expressed as a magnetoresistance ratio in percent.
Conventional materials (e.g., permalloy) typically have a positive MR ratio of a few percent. Recently, relatively large values of MR ratio were observed in metallic multilayer structures, e.g., Fe/Cr or Cu/Co, e.g., as high as 40% at room temperature. See, for instance, E. F. Fullerton, Applied Physics Letters, Vol. 63 (12), p. 1699. High MR ratios at low temperature (typically below 50K) were also observed in Eu.sub.1-x Gd.sub.x Se. Values of MR substantially larger (in absolute value) than those of conventional materials are usually referred to as "giant" MR. See, for instance, P. M. Levy, Science, Vol. 256, p. 972.
More recently still, giant MR ratios were observed in mixed metal oxides. K. Chahara et al., Applied Physics Letters, Vol. 63 (14), pp. 1990-1992 disclose a ratio of -53% in La.sub.0.72 Ca.sub.0.25 MnO.sub.z. The material exhibited essentially zero MR at room temperature.
R. von Helmholt et al., Physical Review Letters, Vol. 71 (14), pp. 2331-2333 report observation of a room temperature MR ratio of about -150% in thin films of perovskite-like La.sub.0.67 Ba.sub.0.33 Mn-oxide. As-deposited films were paramagnetic, but after subsequent heat treatment (900.degree. C., air, 12 hours) the samples exhibited a ferromagnetic magnetization curve. Bulk samples of that composition are known to be metallic ferromagnets, with Curie temperature of 343K, but small MR ratio.
It will be readily apparent that for purposes of commercial use, it would be highly desirable to have available a material which exhibits even higher MR at room temperature (.about.25.degree. C.). A higher MR ratio could lead, inter alia, to increased device sensitivity, and potentially make possible new applications. The possibility of room temperature operation would greatly simplify device design, resulting in considerable cost saving and, potentially, enhanced reliability. This application discloses mixed metal oxides that can exhibit a MR ratio of 200% or more (in absolute value) at room temperature. The application also discloses a method of making such material.